The present invention relates to phase detectors for Non-Return-to-Zero (NRZ) data.
One type of phase detector is the Hogge phase detector, which is described in U.S. Pat. No. 4,535,459. The Hogge phase detector provides good performance by detecting the phase at the mid-point of a pulse, where there is maximum noise immunity.
The above-referenced co-pending application modifies a Hogge phase detector for use in a Return-to-Zero (RZ) phase detector. The present invention, on the other hand, is directed to a different modification for improved use with NRZ data.
NRZ data has recently been popularized in the synchronous optical network (SONET) protocol used in fiber optics. The Hogge detector produces pump-up and pump-down signals which control the charge in the charge pump to provide a correction signal used in tracking and synchronizing with the incoming data. As clock speeds become faster, the pump-up and pump-down signals are shorter, leading to potential errors. One method for addressing this is to divide down the clock signal. However, this generates a separate problem with clock skew. An additional problem with high-speed data is that the clock-to-data output delay through the flip-flops used in a Hogge detector become significant with respect to the clock period. An example of a patent which discusses and addresses clock to data delay is Pat. No. 6,316,966.
Accordingly, it would be desirable to have a circuit which could implement a Hogge detector in a manner which can handle high-speed data without errors, and without clock skew.